XXXIX. Structure and Phase Transitions of Ferroelectric Sodium-Lead Niobates and of other Sodium Niobate Type Ceramics

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Abstract
A structural and dielectric study of ceramics based on sodium niobate has shown that solid solutions of perovskite type are formed between NaNbO3 and PbNb2O6 up to a limit of about 35% replacement of sodium atoms by lead atoms and vacancies. A new ferroelectric tetragonal phase is introduced immediately below the Curie temperature. At lower temperatures the ferroelectric properties become weaker and the structure is orthorhombic with a multiple unit cell of the type found with NaNbO3. For compositions in which more than 25% of the sodium atoms are replaced the room temperature structure is tetragonal. Compositions near the limit of solid solution possess a simple monomolecular perovskite-type cell. Partial substitution of niobium by tantalum lowers the ferroelectric transition temperatures. Comparison of the properties of NaNbO3 with those of solid solutions with PbNb2O6, CdNb2O8 and KNbO3 shows that multiple cell effects can be ascribed to puckering of the niobium-oxygen framework, and depend primarily on lattice spacing and packing. The Curie temperature appears to be determined by characteristics of the replacement atom other than its size. The structure below the Curie temperature depends on the puckering and on the nature of the ferroelectric and antiferroelectric coupling.